The present invention relates to composite articles formed of two or more inorganic materials of differing composition and properties, and more particularly to so-called "microlaminated" composites formed by the lamination of sheets of such inorganic materials.
Composites are often made to create a beneficial mix of the properties of dissimilar materials that can be unobtainable in a single homogeneous material. Two well-known examples of a type of composite structure using a layering of two materials to obtain such a mix of properties are "Samurai Swords" and "Damascus Steel". In these composites, hard but brittle carbide layers are interspersed with softer, ductile, tougher steel layers. In the Samurai case, these layers are created by repeated hammering and folding during the manufacture of the sword.
Newer varieties of metal matrix (ductile) composites reinforced with metallic and non-metallic fibers, whiskers, platelets and particles have also been developed. The reinforcing agents in these composites are intended to impart greater stiffness, higher yield strength or ultimate tensile strength, and/or higher creep resistance to the matrix material. Various methods have been employed to fabricate such composites. In the case of chopped fibers whiskers, platelets, or the like, mixtures of components may be melt processed or sintered to incorporate the reinforcing agents. Long fibers can be laid between metal sheets and the metal sheets deformed around the fibers to obtain a densified composite, although for large fibers a great deal of plastic deformation is necessary to make the matrix material flow around the fiber to make a dense matrix.
Intermetallic compounds are also topics of composites research and, although classed as semi-brittle materials, offer some advantages over conventional alloys. However, while they can be reinforced with ceramic materials, difficulties due to limited ductility and thermodynamic incompatibility with ceramic reinforcing materials are more common, particularly where substantial plastic deformation of the intermetallic is required during manufacture.
Brittle materials such as glass and ceramics are even harder to process, crystalline ceramics being particularly difficult to manufacture to net shape at precise tolerances. Thus ceramics and glass have been largely limited to low stress applications or to areas where their properties (optical transparency, high dielectric constant, etc.) are essential.
Toughened fiber-reinforced ceramic matrix composites for high-stress applications have recently been developed, but the high pressure/high temperature consolidation processing needed for the manufacture of these composites greatly adds to their cost, and the achievement of a truly uniform distribution of reinforcing fibers therein remains difficult.
In broad aspect, composites can be classified into brittle/brittle, brittle/semibrittle, and brittle/ductile categories, depending upon the particular combinations of brittle materials (eg., ceramics) and ductile materials (eg., metals) used in their construction. Layered or laminated composites, familiar in products such as capacitors and multilayer electronic substrates, comprise a well-recognized subgroup of composite structures.
The physical processes involved in the manufacture of well-bonded laminated composites include co-sintering, hot pressing, metal infiltration, and diffusion bonding. Co-sintering is employed in the fabrication of ceramic/ceramic and metal/ceramic composites, with metal infiltration and diffusion bonding also being employed for the latter. For very thin layers of ceramic or metal, vapor deposition processes including ion plating and/or plasma spraying have been employed.
Laminar composites incorporating glass layers can be bonded by hot pressing or glass sintering at temperatures where the glass can wet adjoining materials. In fact, some composites include powdered glass simply as a sealing material to bond otherwise incompatible metallic or ceramic laminae into a unitary structure. U.S. Pat. Nos. 4,868,711 (Hirama et al.) and 3,490,887 (Herczog et al.) offer examples of glass-containing composites.
In the prior art, ceramic/metal laminates have quite often used relatively thick ceramic laminae. For example, Cao and Evans, in "On Crack Extension in Ductile/Brittle Laminates", Acta metall. mater., Vol. 39, No. 12 pp. 2997-3005 (December 1991), have described alumina/aluminum composites comprising 1 mm commercial IC substrate alumina sheets as ceramic layers.
Thinner ceramic layers have been produced in ceramic capacitors, electronic substrates, and similar composites by co-sintering. For example, Marshall et al. in "Enhanced Fracture Toughness in Layered Microcomposites of Ce-ZrO2 and Al2O3", J. Am. Ceram. Soc., 74 [12] 2979-87 (1991), describe laminar composites of Ce-ZrO2 and Ce-ZrO2+Al2O3 with layers on the order of 10-100 micrometers in thickness using colloids with sequential centrifugation and co-sintering.
Unfortunately, ceramic/ceramic and ceramic/metal co-sintering processes present serious obstacles to the attainment of truly homogenous and defect-free composite structures. Such obstacles include relatively crude and non-uniform layer structures, curling or wrinkling of the structures during the co-sintering process, and pin-hole or other layering defects. In the case of metal/ceramic composites, for example, the metal powder and the ceramic powder must sinter in nearly the same temperature range, and the densification shrinkages of the ceramic and metal cannot be too different. Also, kinetically and thermodynamically stable combinations of materials should be used, with compatible partial pressures of gaseous species (PO2 and PH2O) being maintained during sintering.
The manufacture of metal/ceramic composites for capacitors provides a practical example of co-sintering as presently practiced. In that process, ceramic powders and metallic powders are combined, eg., by tape casting the ceramic layer and screen printing the metal layer, and the resulting laminae are stacked and heated to remove binders and solvents. Thereafter, the debindered stacked structures are heated to sinter the metal and ceramic materials. However, before sintering is attempted the stacks are first diced into relatively small chip sizes as required for capacitor use.
While satisfactory for capacitor fabrication, the manufacture of mechanically durable composites from continuous sheets of unsintered powder materials is not practical. This is primarily due to the very high shrinkage factors involved. Even with relatively well matched ceramic and metallic starting materials, extensive layer shrinkage during powder sintering favors the formation of multiple layer defects, eg., pin-hole defects as in metal capacitor films. In addition, large scale layer fluctuations and laminate distortions can occur, as previously noted. These kinds of defects are not significant in products such as capacitors, which are generally below 1 cm size and which can tolerate a relatively high number of process-induced pin-hole defects in the metal layers. Where the mechanical properties of the composites are key, however, such defects are not acceptable.
It is therefore a principal object of the present invention to provide an improved process for the fabrication of microlaminated composites which provides well-bonded laminar articles with improved layer structure.
It is a further object of the invention to provide microlaminated composites offering improved physical integrity and structural uniformity.
Other objects and advantages of the invention will become apparent from the following description thereof.